A professional baseball player before he became a country star, Keith Anderson is a competitive guy ... and a prankster. When The Boot ran into the singer/songwriter at Wednesday's City of Hope Celebrity Softball Challenge in Nashville, he was asking our buddies at Country Music Tattle Tale to borrow a sharpie. Why? So he could draw a mustache on Carrie Underwood -- a poster of her, that is.

"Y'all don't tell her I did this," Keith said to us with a wink, as he drew a handlebar mustache on the picture of Carrie.

Keith has nothing against Carrie -- she just happened to be on the Grand Ole Opry team, while he was on the opposing 'After Midnight' team at the annual game, which kicks off CMA Music Fest. (See pictures from the game here.)

"I'm very competitive -- I used to play baseball," Keith told The Boot just before the game. "I better not strike out!"

Keith didn't strike out, but Carrie did get the last laugh. She and her Opry teammates won the game 11-5. Hopefully Keith will have better luck at the autograph signings, which he says are his favorite part of Music Fest.

"My first year (of Music Fest) was so funny because nobody had a clue ... half the time they thought I was somebody else," Keith tells The Boot. "You can walk through that room and people will follow you even if they don't know who you are. You quickly find out it's just a bunch of country fans who want to be here and be around country music, which is the coolest thing in the world. It doesn't have to be about you -- it's about country music. I love it. I wouldn't be here if they weren't buying the records and listening to the songs. I love getting to know them."

Keith's annual fan club party will be held Friday, followed by his performance on the Riverfront Stages.